Organic light emitting diodes (OLEDs) contain an organic emissive layer (EML) that emits light by fluorescence or phosphorescence. OLEDs exhibiting highest luminance efficacy and power conversion efficiency are fabricated using electroluminescent materials containing heavy metal atoms. Therefore electroluminescent materials based on transition metal complexes attract widespread attention and enormous amount of research is conducted worldwide in order to improve existing devices. For example, electroluminescent materials comprising complexes with platinum group metals such as platinum (Pt), osmium (Os), ruthenium (Ru) and iridium (Ir) can be used to form an electroluminescent layer in OLEDs, wherein the iridium complexes exhibit the highest efficiency and widest color tuning capability. Iridium complexes exhibiting high luminance efficiency typically have an octahedral structure with the iridium center in a +3 oxidation state. The mechanism of light emission of these iridium complexes is based on a triplet metal to ligand charge transfer (3MLCT) transition between the metal and the ligand, or a triplet ligand-centered (3π−π*)transition. Most often the transition responsible for the light emission in transition metal complexes is of mixed LC-MLCT character. The strong spin-orbit coupling induced by the heavy metal center produces high phosphorescence efficiency by efficiently mixing the singlet and triplet states rendering possible and efficient the formally forbidden transition from the triplet-excited state to the singlet-ground state.
One of the best known triplet-state blue phosphorescent complex is the iridium(III) complex Bis(4,6-difluorophenylpyridinato-N,C2)picolinatoiridium (FIrpic), which is already described in various prior references, such as WO 02/15645 and Appl. Phys. Lett. 2001, 79, 2082. However, FIrPic is not sufficiently stable under the operation conditions of typical OLED devices to allow practical commercial applications.

Some references disclose derivatives of FIrpic. In the specification of PCT Publication No. WO05097943 A1, several iridium complexes are disclosed, e.g., complexes I-74 to I-78 on page 32 of the reference. U.S. Pat. No. 7,534,506 and EP Publication No. EP2036907 A1 also relate to metal complexes for organic electroluminescence devices which are substituted by various electron donating and withdrawing groups.
US 2006/0014047 relates to luminescent devices comprising an organic compound layer comprising a metal coordination compound of the formula

wherein M may be Ir, Rh or Pd.
EP 2036907 relates to organoluminiscent devices and discloses Pt-complexes of formulae

However, none of the above-disclosed materials meets all the requirements necessary for OLED application, particularly thermal/environmental stability which can affect the whole life time of OLED devices, as well as high phosphorescent efficiency. Thus, there has been a need to develop new emissive materials, which are capable of satisfying all of the requirements indicated above.